Aluminosilicate glasses for ion exchange

ABSTRACT

Glass compositions that may be used to produce chemically strengthened glass sheets by ion exchange. The glass compositions are chosen to promote simultaneously high compressive stress and deep depth of layer or, alternatively, to reduce the time needed to ion exchange the glass to produce a predetermined compressive stress and depth of layer.

This application is a continuation of and claims the benefit of priorityto U.S. patent application Ser. No. 13/408,169 (now granted as U.S. Pat.No. 9,701,580), filed on Feb. 29, 2012, the content of which is reliedupon and incorporated herein by reference in its entirety.

BACKGROUND

The disclosure relates to ion exchangeable glasses. More particularly,the disclosure relates to ion exchangeable glasses that, when ionexchanged has a surface layer that is under a compressive stress of atleast about 1 GPa.

The ion exchange process is used to strengthen glass by creating acompressive stress at the glass surface by replacing of relatively largealkali ions such as K⁺ from a salt bath with smaller alkali ions such asNa⁺ in the glass. Since glasses typically fail under tension, thecreated compressive stress at the surface improves the glass strength.Ion exchanged glasses thus find use in various applications such astouch-screen devices, hand held electronic devices such as communicationand entertainment devices, architectural and automotive components, andthe like.

When strengthened by ion exchange, a glass should simultaneously beprovided with high compressive stress at the surface and a deep depth ofthe ion exchange layer. Soda-lime glasses are difficult to chemicallystrengthen by ion exchange as they require long salt bath treatmenttimes to achieve reasonable strength by ion exchange.

SUMMARY

The present disclosure provides glass compositions that may be used toproduce chemically strengthened glass sheets by ion exchange. The glasscompositions are chosen to promote simultaneously high compressivestress and deep depth of layer or, alternatively, to reduce the timeneeded to ion exchange the glass to produce a predetermined compressivestress and depth of layer.

Accordingly, one aspect of the disclosure is to provide an alkalialuminosilicate glass. The alkali aluminosilicate glass comprises fromabout 14 mol % to about 20 mol % Al₂O₃ and from about 12 mol % to about20 mol % of at least one alkali metal oxide R₂O selected from the groupconsisting of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O, wherein the alkalialuminosilicate glass is ion exchangeable.

A second aspect of the disclosure is to provide an alkalialuminosilicate glass. The alkali aluminosilicate glass comprises fromabout 55 mol % to about 70 mol % SiO₂; from about 14 mol % to about 20mol % Al₂O₃; from 0 mol % to about 10 mol % B₂O₃; from 12 mol % to about20 mol % R₂O, where R₂O is selected from the group consisting of Li₂O,Na₂O, K₂O, Rb₂O, and Cs₂O; from 0 mol % to about 10 mol % MgO; and from0 mol % to about 10 mol % ZnO. The alkali aluminosilicate glass is ionexchanged and has a compressive layer extending from a surface of thealkali aluminosilicate glass into the alkali aluminosilicate glass to adepth of layer. The compressive layer is under a compressive stress ofat least 1 GPa.

These and other aspects, advantages, and salient features will becomeapparent from the following detailed description, the accompanyingdrawings, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a plot of compressive stress as a function of [Al₂O₃]—[R₂O];

FIG. 2 is a plot of depth of layer (DOL) as a function of[Al₂O₃]—[Na₂O];

FIG. 3 is a plot of compressive stress (CS) for a fixed depth of layerof 50 μm as a function of [MgO]/([MgO]+[CaO]) ratio;

FIG. 4 is a plot of the diffusion coefficient D_(Na-K) as a function ofcomposition of the boroaluminosilicate series of glasses describedherein;

FIG. 5 a plot of the composition dependence of isothermal diffusivityand iron redox ratio;

FIG. 6 is a plot of compressive stress (CS) of both Fe-free andFe-containing boroaluminosilicate glasses as a function of composition;

FIG. 7 a plot of the loading and penetration depth condition of theexperiment performed on iron-free boroaluminosilicate glass A117.5 inTable 6; and

FIG. 8 is a plot of compositional dependence of nanohardness (H_(nano))at 98 mN load force for iron-containing and iron-freeboroaluminosilicate glasses.

DETAILED DESCRIPTION

In the following description, like reference characters designate likeor corresponding parts throughout the several views shown in thefigures. It is also understood that, unless otherwise specified, termssuch as “top,” “bottom,” “outward,” “inward,” and the like are words ofconvenience and are not to be construed as limiting terms. In addition,whenever a group is described as comprising at least one of a group ofelements and combinations thereof, it is understood that the group maycomprise, consist essentially of, or consist of any number of thoseelements recited, either individually or in combination with each other.Similarly, whenever a group is described as consisting of at least oneof a group of elements or combinations thereof, it is understood thatthe group may consist of any number of those elements recited, eitherindividually or in combination with each other. Unless otherwisespecified, a range of values, when recited, includes both the upper andlower limits of the range as well as any ranges therebetween. As usedherein, the indefinite articles “a,” “an,” and the correspondingdefinite article “the” mean “at least one” or “one or more,” unlessotherwise specified. As used herein, the term “glass” refers to alkalialuminosilicate and/or boroaluminosilicate glasses, unless otherwisespecified.

Referring to the drawings in general and to FIG. 1 in particular, itwill be understood that the illustrations are for the purpose ofdescribing particular embodiments and are not intended to limit thedisclosure or appended claims thereto. The drawings are not necessarilyto scale, and certain features and certain views of the drawings may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

This disclosure relates to the general area of ion exchangeable alkalialuminosilicate glasses that are capable of—or have been strengthenedby—ion exchange. The ion exchange process is used to create acompressive stress at the glass surface by replacement of relativelylarge alkali ions from a salt bath (e.g., K⁺) with smaller alkali ions(e.g., Na⁺) in the glass. Since glasses typically fail under tension,the compressive stress created at the surface improves the glassstrength. Ion exchanged glasses thus find various applications, such asfor touch-screen devices, hand held electronic devices such ascommunication and entertainment devices, architectural and automotivecomponents, and the like.

Ion exchangeable glass compositions should be designed so as tosimultaneously provide a high compressive stress (CS) at the surface anda deep depth of the ion exchange layer (depth of layer, or “DOL”).Soda-lime glasses are typically difficult to chemically strengthen byion exchange, as they require long salt bath treatment times to achievereasonable strength by such exchange.

The various glass compositions described herein could be used to producechemically strengthened glass sheets by ion exchange. These glasscompositions are chosen to promote simultaneously high compressivestress and deep depth of layer or, alternatively, reduced ion exchangetime. The glass compositions described herein are not necessarily fusionformable or down drawable (e.g., fusion drawn or slot drawn), and may beproduced using other forming methods known in the art; e.g., the floatglass process.

The glasses described herein are ion exchangeable alkali aluminosilicateglasses comprising from about 14 mol % to about 20 mol % Al₂O₃ and fromabout 12 mol % to about 20 mol % of at least one alkali metal oxide R₂Oselected from the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O.In some embodiments, the at least one alkali metal oxide includes Na₂O,and Al₂O₃ (mol %)-Na₂O (mol %)≥about −4 mol %.

In some embodiments, the glasses described herein, when strengthened byion exchange, have a region that is under a compressive stress(compressive layer CS) that extends from the surface of the glass to adepth of layer (DOL) into the body of the glass. The compressive stressof the strengthened glass is at least about 1 GPa. In some embodiments,the compressive stress is at least about 1 GPa and Al₂O₃ (mol %)-Na₂O(mol %)≥about −4 mol %.

In some embodiments, the glass comprises: from about 55 mol % to about70 mol % SiO₂; from about 14 mol % to about 20 mol % Al₂O₃; from 0 mol %to about 10 mol % B₂O₃; from 0 mol % to about 20 mol % Li₂O; from 0 mol% to about 20 mol % Na₂O; from 0 mol % to about 8 mol % K₂O; from 0 mol% to about 10 mol % MgO; and from 0 mol % to about 10 mol % ZnO. Inparticular embodiments, 12 mol % Li₂O+Na₂O+K₂O≤20 mol %.

In one aspect, the alkali aluminosilicate glasses are sodiumaluminosilicate glasses that further comprise different types ofdivalent cation oxides RO, also referred to herein as “divalent metaloxides” or simply “divalent oxides” in which the silica-to-alumina ratio([SiO₂]/[Al₂O₃]) is not fixed, but may instead be varied. These divalentmetal oxides RO include, in one embodiment, MgO, ZnO, CaO, SrO, and BaO.Non-limiting examples of such compositions having the general formula(76-x) mol % SiO₂, x mol % Al₂O₃, 16 mol % Na₂O, and 8 mol % RO, inwhich x=0, 2.7, 5.3, 8, 10.7, 13.3, 16, 18.7, 21.3, and 24 andproperties associated with each composition are listed in Tables 1, 2,and 3, for R=Mg, R=Zn, and R=Ca, respectively. Non-limiting examples ofsuch compositions, expressed in mol % where(76-x)SiO₂-xAl₂O₃-16Na₂O-8RO, where x=0, 8, 16, and 24, and propertiesassociated with such compositions for R=Sr and Ba are listed in Table 5.For x=16, four glasses with [MgO]/[CaO] ratios equal to 0.25, 0.67, 1.5,and 4 were also studied, in addition to glasses with K₂O-for-Na₂Osubstitutions and higher SiO₂ contents (Table 4). In some embodiments,these glasses are free of (i.e., contain 0 mol %) boron andboron-containing compounds, such as, for example, B₂O₃.

In other embodiments, the alkali aluminosilicate glasses describedherein are boroaluminosilicate glasses comprising up to about 10 mol %B₂O₃ with varying silica-to-alumina ratios. These boroaluminosilicateglasses may, in some embodiments, be free of (i.e., contain 0 mol %)divalent metal oxides RO, such as those described hereinabove.Non-limiting examples of such boroaluminosilicate glasses having nominalcompositions, expressed in mol % of: (80-y) mol % SiO₂, y mol % Al₂O₃,15 mol % Na₂O, and 5 mol % B₂O₃, where y=0, 1, 2.5, 5, 7.5, 10, 12.5,15, 17.5, and 20, and associated properties are listed in Table 6.

In the glass compositions described herein, SiO₂ serves as the primaryglass-forming oxide. The concentration of SiO₂ should be sufficientlyhigh in order to provide the glass with sufficiently high chemicaldurability suitable for touch applications. However, the meltingtemperature (i.e., the 200 poise temperature) of pure SiO₂ or high-SiO₂glasses is too high to be practical for most manufacturing processes,since defects such as fining bubbles may appear. Furthermore, whencompared to every oxide except boron oxide (B₂O₃), SiO₂ decreases thecompressive stress created by ion exchange.

Alumina (Al₂O₃) can also serve as a glass former in the glassesdescribed herein. Like SiO₂, alumina generally increases the viscosityof the melt and an increase in Al₂O₃ relative to the alkalis or alkalineearths in the glass generally results in improved durability. Thestructural role of aluminum ions depends on the glass composition. Whenthe concentration of alkali metal oxides [R₂O] is greater than theconcentration of alumina [Al₂O₃], all aluminum is primarily found intetrahedral coordination with the alkali ions acting ascharge-balancers. For [Al₂O₃]>[R₂O], there is an insufficient amount ofalkali metal oxides to charge balance all aluminum in tetrahedralcoordination. However, divalent cation oxides (RO) can also chargebalance tetrahedral aluminum to varying degrees. Whereas Calcium,strontium, and barium all primarily behave in a manner equivalent to twoalkali ions, the high field strength magnesium and zinc ions do notfully charge balance aluminum in tetrahedral coordination, and result inthe formation of five- and six-fold coordinated aluminum. Al₂O₃generally plays an important role in ion-exchangeable glasses, since itprovides or enables a strong network backbone (i.e., a high strainpoint) while allowing for the relatively fast diffusivity of alkaliions. As evidenced by the plot of compressive stress as a function of[Al₂O₃]—[R₂O] in FIG. 1 for the glass compositions listed in Tables 1-5after ion exchange in technical grade KNO₃ at 410° C. for eight hours,the presence of tetrahedral aluminum promotes a high compressive stress.As seen in FIG. 1, compressive stress CS generally increases withincreasing alumina content and decreasing size of the divalent cation.In the peraluminous regime, there is an advantage from having the largerdivalent cations. Most likely these cations act to charge balancetetrahedral aluminum, whereas the smaller divalent cations in MgO andZnO do not to the same extent. In glasses with excess magnesium andzinc, however, the addition of alumina decreases the depth of thecompressive layer for a given ion exchange time when [Al₂O₃]>[R₂O].

Although B₂O₃ is also a glass-forming oxide, it can be used to reduceviscosity and liquidus temperature. In general, an increase in B₂O₃ of 1mol % decreases the temperature at equivalent viscosity by 10-14° C.,depending on the details of the glass composition and the viscosity inquestion. However, B₂O₃ can lower liquidus temperature by 18-22° C. permol %, and thus has the effect of decreasing liquidus temperature morerapidly than it decreases viscosity, thereby increasing liquidusviscosity. Furthermore, B₂O₃ has a positive impact on the intrinsicdamage resistance of the base glass. However, B₂O₃ has a negative impacton ion exchange performance, decreasing both the diffusivity and thecompressive stress. For example, substitution of SiO₂ for B₂O₃ increasesion exchange performance but simultaneously increases melt viscosity.

Alkali metal oxides (Li₂O, Na₂O, and K₂O) serve as aids in achieving lowmelting temperature and low liquidus temperatures. However, the additionof alkali metal oxides dramatically increases the coefficient of thermalexpansion (CTE) and lowers chemical durability.

The presence of a small alkali metal oxide such as Li₂O and/or Na₂O isnecessary to exchange with larger alkali ions (e.g., K⁺) to perform ionexchange from a salt bath and thus achieve a desired level of surfacecompressive stress in the glass. Three types of ion exchange cangenerally be carried out: Na⁺-for-Li⁺ exchange, which results in a deepdepth of layer but low compressive stress; K⁺ for-Li⁺ exchange, whichresults in a small depth of layer but a relatively large compressivestress; and K⁺ for-Na⁺ exchange, which results in intermediate depth oflayer and compressive stress. A sufficiently high concentration of thesmall alkali metal oxide is necessary to produce a large compressivestress in the glass, since compressive stress is proportional to thenumber of alkali metal ions that are exchanged out of the glass. Thepresence of a small amount of K₂O generally improves diffusivity andlowers the liquidus temperature, but increases the CTE.

Divalent cation oxides RO such as, but not limited to, alkaline earthoxides and ZnO, also improve the melting behavior of the glass. Withrespect to ion exchange performance, however, the presence of divalentcations acts to decrease alkali metal ion mobility. The effect on ionexchange performance is especially pronounced with the larger divalentcations such as, for example, Sr²⁺ and Ba²⁺, as illustrated in FIG. 2,which is a plot of depth of layer (DOL) as a function of [Al₂O₃]—[Na₂O]for ion exchanged glasses having the composition (76-x) mol % SiO₂, xmol % Al₂O₃, 16 mol % Na₂O, and 8 mol % RO, where x=0, 2.7, 5.3, 8,10.7, 13.3, 16, 18.7, 21.3, and 24 for R=Mg (Table 1), Zn (Table 2), andCa (Table 3) and x=0, 8, 16, and 24 for R=Sr and Ba (Table 5). The ionexchange was performed in a molten salt bath of technical grade KNO₃ at410° C. for 8 hours. As seen in FIG. 2, DOL generally decreases withincreasing alumina content, especially for the glasses containing MgOand ZnO in the peraluminous regime. Furthermore, as seen in FIG. 1,smaller divalent cation oxides generally help the compressive stressmore than larger divalent cation oxides. In the glasses describedherein, the concentrations of SrO and BaO are particularly kept to aminimum.

MgO and ZnO offer several advantages with respect to improved stressrelaxation while minimizing the adverse effects on alkali diffusivity.However, when the amounts of MgO and ZnO in the glass are high, theseoxides are prone to forming forsterite (Mg₂SiO₄) and gahnite (ZnAl₂O₄),or willemite (Zn₂SiO₄), thus causing the liquidus temperature to risevery steeply with MgO and ZnO contents. Furthermore, there may be someadvantages from having a mixture of two alkaline earth oxides, asillustrated in FIG. 3, which is a plot of compressive stress (CS) for afixed depth of layer of 50 μm as a function of [MgO]/([MgO]+[CaO]) ratiofor ion exchanged glasses having the composition 60 mol % SiO₂, 16 mol %Al₂O₃, 16 mol % Na₂O, and 8 mol % RO. The glasses were ion exchanged ina molten salt bath of technical grade KNO₃ at 410° C. for differentdurations. As seen in FIG. 3, compressive stress CS at 50 μm generallyincreases with increasing magnesia content, but there is an advantagefrom having a mixture of CaO and MgO in the high-MgO regime.

In addition to the oxides described above, other oxides may be added tothe glasses described herein to eliminate and reduce defects within theglass. For example, SnO₂, As₂O₃, Sb₂O₃, or the like may be included inthe glass as fining agents. Increasing the concentration of SnO₂, As₂O₃,or Sb₂O₃ generally improves the fining capacity, but as they arecomparatively expensive raw materials, it is desirable to add no morethan is required to drive gaseous inclusions to an appropriately lowlevel.

The main forming/stabilizing cations and molecules in silicate meltsinclude Si⁴⁺, Al, B, Fe³⁺, Ti, P, and the like. The main networkmodifying cations and molecules include Na⁺, K+, Ca²⁺, Mg²⁺, Fe²⁺, F⁻,Cl⁻, and H₂O, although their role in defining the structure is oftencontroversial. Iron as Fe³⁺ (ferric iron) can be a network former withcoordination number IV or V and/or a network modifier with coordinationV or VI, depending on the Fe³⁺/ΣFe ratio, whereas Fe²⁺ (ferrous) iron isgenerally considered to be a network modifier. As both ferric andferrous iron can be present in liquids, changes in the oxidation stateof iron can affect significantly their degree of polymerization.Therefore, any melt property that depends on the number of non-bridgingoxygen per tetrahedron (NBO)/T) will also be affected by the ratioFe³⁺/ΣFe. Significant portions of Si and Al may exist in five-foldcoordination at ambient pressure.

In order to explore different structural roles filled by sodium in theboroaluminosilicate glasses, ten Na₂O—B₂O₃—Al₂O₃—SiO₂ glasses withvariation of the [Al₂O₃]/[SiO₂] ratio were designed to access differentregimes of sodium behavior. Ten additional ten glasses having the samebase composition, but doped with 1 mol % Fe₂O₃ were also prepared tostudy the effect of Fe₂O₃ on ion exchange properties. The compositionsof these glasses are designated as x mol % Al₂O₃, 5 mol % B₂O₃, (80-x)mol % SiO₂, and 15 mol % Na₂O, where x=0, 1, 2.5, 5, 7.5, 10, 12.5, 15,17.5, and 20, with the analyzed compositions being slightly differentfrom the batched compositions. The original naming convention based onxAl₂O₃, as given in Table 6, is retained. As a result of this work, thedifferent roles/effects of sodium on the network-forming cations (Si, B,and Al) have been clarified and quantified. When Na<Al, all sodium isused to charge compensate [AlO₄], and [AlO₅] groups, which are alsopresent in the glass and act as charge compensators due to insufficientamounts of sodium in the glass. When Na>Al, sodium first chargecompensates [AlO₄], and all Al is thus four-coordinated and unaffectedby other compositional changes. Excess sodium can be used to convert[BO₃] to [BO₄], or create non-bridging oxygens (NBOs) on Si or B, withcompetition among these mechanisms.

Ion exchange experiments were conducted to obtain the effectiveinterdiffusion coefficient D _(Na-K) between Na⁺ and K⁺ and thecompressive stress (CS) in the glasses described herein. Ion exchangewas carried out by immersing polished 25 mm×25 mm×1 mm glass samples ina molten salt bath of technical grade KNO₃ at 410° C. for 8 hours.Following ion exchange, the penetration depth of the potassium ions wasmeasured using an FSM-6000 surface stress meter (FSM). The K⁺ for-Na⁺ion exchange gives the glass surface a higher refractive index than theinterior; i.e., the surface acts as a waveguide. This is utilized in theFSM instrument to measure the saturation depth of the refractive indexprofile, which corresponds to the diffusion depth of potassium. A totalof eight FSM measurements were performed on each sample (using four 90°rotations per face).

The results of these ion exchange experiments reveal a decrease inalkali diffusivity with increasing [SiO₂]/[Al₂O₃] or [SiO₂]/Σ[Oxi] whereΣ[Oxi]=[SiO₂]+[Al₂O₃]+[B₂O₃]+[Fe₂O₃]+[As2O₃] ratios for bothiron-containing and iron-free glasses. FIG. 4 is a plot of the diffusioncoefficient D_(Na-K) as a function of composition of theboroaluminosilicate series of glasses described herein. The data plottedin FIG. 4 show that the roles of sodium and boron change as the[SiO₂]/[Al₂O₃] ratio changes. This trend may be ascribed to two factors.First, the structural role of sodium in influencing sodium diffusiondepends on the [SiO₂]/[Al₂O₃] ratio. For high Al₂O₃ contents, Na⁺ isused for charge compensation of four-fold aluminum species. In thiscase, the diffusion of Na⁺ is relatively fast, as shown in FIG. 5, whichis a plot of the composition dependence of isothermal diffusivity (K⁺for-Na⁺ effective interdiffusion coefficient (D _(Na-K))), as determinedby ion exchange experiments at 410° C., and the iron redox state, asdetermined by ⁵⁷Fe Mössbauer spectroscopy. This fast diffusion rate ofNa may be because Na⁺ is not a rigid part of the glass network. In thelow Al₂O₃ composition region, some of the sodium ions create NBOs bondedwith Si—O or B—O, and these sodium ions are less mobile. Secondly, thedifferences in boron speciation and chemical composition lead todifferences in atomic packing of the glass networks. The network becomesmore densely packed with increasing [SiO₂]/[Al₂O₃] ratio, and thiscontributes to the lowering of the alkali diffusivity. FIG. 5 alsoreveals that the alkali diffusivity is greater in iron-free glasses thanin iron-containing glasses. Furthermore, the difference in alkalidiffusivity between iron-free and iron-containing glasses decreases withincreasing [SiO₂]/[Al₂O₃] ratio while at the same time the[Fe³⁺]/[Fe]_(total) ratio increases (see the second y-axis in FIG. 5)).Therefore, Fe²⁺ is a greater hindrance to alkali diffusivity than Fe³⁺.In other words, there is little or no decrease in alkali diffusivitywhen iron is present as Fe³⁺. The impact of iron on alkali diffusivitymay be ascribed to two factors. First, there is competition betweencations for the charge compensation of AlO₄ ⁻ and BO; units. It has beenshown that Fe²⁺ can charge compensate AlO₄ ⁻ units in aluminosilicateglasses, even though alkali ions are more efficient charge compensatorsthan Fe²⁺. It is therefore possible that some Fe²⁺ ions can compete withNa+ ions for charge compensating AlO₄ ⁻ (and possibly also BO₄ ⁻), whichcould cause some of the sodium ions to create NBOs on tetrahedralsilicon or trigonal boron. According to the discussion above, this willlower the alkali diffusivity. Second, the presence of relatively slowlymoving divalent cations lowers the mobility of the fast movingmonovalent alkali cations. Fe²⁺ ions play a role as network-modifiers inthe glass network, and may therefore be blocking the diffusion paths ofthe fast moving Na⁺ ions (similar to the impact of alkaline earth ionson alkali diffusivity). On the other hand, Fe³⁺ ions play a morenetwork-forming role in the network, and they are therefore notoccupying sites that Na⁺ ions would use for diffusion.

FIG. 6 is a plot of compressive stress (CS) of both Fe-free andFe-containing boroaluminosilicate glasses as a function of composition(i.e., [Al₂O₃]—[Na₂O]). CS was measured by FSM on the annealed samples,which were chemically strengthened in a molten salt bath of technicalgrade KNO₃ salt bath at 410° C. for 8 hours. As shown in FIG. 6,compressive stress created by ion exchange was found to monotonicallyincrease with increasing Al₂O₃ concentration in the boroaluminosilicateglasses. This finding is in agreement with that reported above for thesodium aluminosilicate glasses with different divalent cations. Theiron-containing glasses were also found to generally have higher CS thanthe corresponding iron-free glasses, particularly in the peralkalineregime.

Additionally, eight hardness measurements using the nano-indentationtechnique for each composition were also performed on some of theglasses described herein. The hardness values reported in Table 6 werecalculated from indentation depths ranging from 598 nm to 998 nm. FIG. 7is a plot of the loading and penetration depth condition of theexperiment performed on sample Al17.5, which is listed in Table 6. Thecompositional dependence of nano-hardness (H_(nano)) at 98 mN load forcefor both iron-containing and iron-free boroaluminosilicate glasses isplotted in FIG. 8. The gray and black solid symbols in FIG. 8 representglasses before and after ion exchange, respectively, which were ionexchanged at 410° C. for 8 hours in a technical grade KNO₃ molten saltbath. The nano-indentation hardness technique does not reveal largedifferences in hardness for the iron-free and iron-containing glasses,neither before nor after being chemically strengthened in the KNO₃ saltbath at 410° C. for 8 hours. In some embodiments, the glasses describedherein have a nanohardness of at least about 7 GPa after ion exchanged.Nonetheless, the ion exchanged peraluminous (Al>Na) glass end membersexhibit a systematic increase in nano-hardness of about 1.5 GPa comparedto glasses without a chemically strengthened surface. The peralkaline(Al<Na) ion exchanged end members also show an increase in nano-hardnesswhen compared with glasses without chemically strengthened surfaces, butthe difference is only about 0.5 GPa. This may be due to the lowercompressive stresses created in these peralkaline compositions (FIG. 6).

While typical embodiments have been set forth for the purpose ofillustration, the foregoing description should not be deemed to be alimitation on the scope of the disclosure or appended claims.Accordingly, various modifications, adaptations, and alternatives mayoccur to one skilled in the art without departing from the spirit andscope of the present disclosure or appended claims.

TABLE 1 Examples of ion-exchangeable glass compositions containing MgO.The compressive stress (CS) and depth of layer (DOL) were obtained as aresult of treatment of annealed samples at 410° C. for 16 hours intechnical grade KNO₃. Composition (mol %) 1 2 3 4 5 6 7 8 9 10 SiO₂75.83 73.70 70.88 68.07 65.33 62.77 59.92 56.62 54.64 52.02 A1₂O₃ 0.072.71 5.32 7.99 10.72 13.31 15.98 18.63 21.33 23.97 Na₂O 15.63 15.7315.68 15.71 15.74 15.78 15.77 15.55 15.78 15.82 K₂O MgO 8.11 7.62 7.887.98 7.95 7.90 8.08 8.94 7.99 7.93 ZnO CaO 0.19 0.07 0.09 0.09 0.09 0.080.09 0.09 0.09 0.09 SrO BaO SnO₂ 0.16 0.16 0.16 0.16 0.16 0.15 0.16 0.160.16 0.16 Properties 1 2 3 4 5 6 7 8 9 10 Anneal Pt (C): 507 541 578 617652 683 698 712 723 732 Strain Pt (C): 462 495 530 568 601 632 648 662674 685 Density 2.415 2.424 2.437 2.448 2.461 2.47 2.49 2.501 2.5132.527 (g/cm{circumflex over ( )}3): CTE (×10{circumflex over ( )}- 87.486.2 86.1 85.8 84.1 82.80 80.50 76.2 74.5 70.3 7/C): Softening Pt 708.7748.7 791.8 836.1 875.1 909.40 924.90 936.8 939.7 938.6 (C): 24-hLiquidus 985 no no no 1180 >1250 1250 >1385 >1385 >1385 (C): devit devitdevit Primary Devit tridymite forsterite forsterite forsteriteforsterite unknown unknown Phase: Liquidus Visc 30364 21433 11390 <1517<1010 <813 (Poise): Poisson's 0.212 0.215 0.204 0.219 0.206 0.22 0.220.215 0.225 0.22 Ratio: Shear 27.59 28.06 28.70 29.22 30.00 30.54 31.1631.92 32.66 33.38 Modulus (GPa): Young's 66.89 68.20 69.09 71.24 72.3674.44 76.16 77.59 80.03 81.46 Modulus (GPa): Refractive 1.4971 1.49921.5011 1.5034 1.5061 1.5090 1.5123 1.5160 1.5196 1.5234 Index: SOC 28.4928.66 28.54 28.67 28.75 28.60 28.36 27.96 27.5 27.14 (nm/cm/MPa): CS(MPa): 128 441 663 876 1062 1154 1192 1166 1124 1056 DOL (μm): 44.1948.63 47.90 46.45 46.33 43.88 39.59 32.26 24.96 19.21

TABLE 2 Examples of ion-exchangeable glass compositions containing ZnO.The compressive stress (CS) and depth of layer (DOL) were obtained as aresult of treatment of annealed samples at 410° C. for 16 hours intechnical grade KNO₃. Composition (mol %) 11 12 13 14 15 16 17 18 19 20SiO₂ 76.35 73.53 71.04 68.24 65.50 62.91 60.03 57.34 54.70 52.01 Al₂O₃0.02 2.72 5.34 8.03 10.74 13.38 16.02 18.80 21.36 24.05 Na₂O 15.42 15.6115.61 15.64 15.57 15.74 15.62 15.79 15.66 15.74 K₂O MgO ZnO 8.06 7.987.86 7.93 8.03 7.82 8.17 7.92 8.12 8.04 CaO SrO BaO SnO₂ 0.15 0.15 0.150.15 0.15 0.15 0.15 0.15 0.15 0.15 Properties 11 12 13 14 15 16 17 18 1920 Anneal Pt (C): 513 544 577 609 639 658 673 684 696 708 Strain Pt (C):467 497 528 562 589 609 625 635 647 660 Density 2.541 2.558 2.566 2.5702.581 2.585 2.600 2.611 2.623 2.636 (g/cm{circumflex over ( )}3): CTE(×10{circumflex over ( )}- 86.4 85.6 85.7 84.8 83.4 81.8 78.3 75.8 71.768.9 7/C): Softening Pt 706 742 779 819 857 885 902 909 910 915 (C):24-h Liquidus 1040 no devit 870 935 no 1070 1370 >1390 >1390 >1390 (C):devit Primary Devit Tridy Albite Albite Unknown Spinel Unknown UnknownUnknown Phase: mite Liquidus Visc 191253 87234 19996 (Poise): 8634 4 2 01849 <1231 <909 <10 Poisson's 0.218 0.214 0.216 0.217 0.223 0.22 0.230.228 0.226 0.24100 Ratio: Shear 27.02 27.78 28.65 29.04 29.50 30.2130.77 31.53 32.31 32.96 Modulus (GPa): Young's 65.81 67.48 69.00 70.6972.13 73.79 75.50 77.41 79.22 81.82 Modulus (GPa): Refractive 1.5081.514 1.516 1.534 1.524 Index: 0 1.5102 1.5123 1 1 5 1.5215 7 1.52861.5325 SOC 33.08 32.94 32.99 32.74 32.22 31.65 31.01 30.36 29.67 29.10(nm/cm/MPa): CS (MPa): 467 659 872 1070 1134 1186 1165 1123 1023 DOL(μm): 50.01 49.50 47.01 45.57 44.24 39.31 32.32 25.63 19.65

TABLE 3 Examples of ion-exchangeable glass compositions containing CaO.The compressive stress (CS) and depth of layer (DOL) were obtained as aresult of treatment of annealed samples at 410° C. for 16 hours intechnical grade KNO₃. Composition (mol %) 21 22 23 24 25 26 27 28 29 30SiO₂ 75.88 73.19 70.73 68.08 65.20 62.58 59.83 57.18 54.26 51.82 Al₂O₃0.03 2.71 5.30 8.02 10.72 13.29 16.01 18.71 21.34 23.97 Na₂O 15.72 15.7615.78 15.72 15.77 15.80 15.79 15.68 15.70 15.81 K₂O MgO 0.10 0.10 0.110.09 0.12 0.12 0.13 0.13 0.13 0.13 ZnO CaO 8.11 8.10 7.91 7.92 8.03 8.058.08 8.15 8.40 8.11 SrO BaO SnO₂ 0.16 0.16 0.16 0.16 0.16 0.16 0.16 0.150.16 0.16 Properties 21 22 23 24 25 26 27 28 29 30 Anneal Pt (C): 525548 567 591 619 647 678 710 738 756 Strain Pt (C): 483 505 524 547 574601 630 661 690 709 Density 2.474 2.485 2.491 2.499 2.509 2.52 2.522.528 2.537 2.547 (g/cm{circumflex over ( )}3): CTE (×10{circumflex over( )}- 92.5 90.9 89.4 88.2 87.7 87.00 85.40 82.8 80.2 77.3 7/C):Softening Pt 700.2 725.1 749.9 779.5 811 845.00 882.10 921.5 (C): 24-hLiquidus 990 900 990 1070 1250 1250 1155 1245 1300 1295 (C): PrimaryDevit tridymite devitrite devitrite devitrite anorthite anorthitenepheline nepheline unknown unknown Phase: Liquidus Visc 10657 2165 370429476 10457 4955 4573 (Poise): Poisson's 0.212 0.212 0.223 0.221 0.2230.23 0.22 0.237 0.238 0.221 Ratio: Shear 28.78 29.28 29.70 30.10 30.5330.91 31.25 31.62 32.16 32.82 Modulus (GPa): Young's 69.75 71.00 72.6273.48 74.66 75.70 76.47 78.24 79.64 80.17 Modulus (GPa): Refractive1.5119 1.5138 1.5150 1.5166 1.5183 1.5198 1.5218 1.5235 1.5259 1.5292Index: SOC 27.33 27.41 27.5 27.49 27.36 27.39 27.45 27.35 27.11 26.71(nm/cm/MPa): CS (MPa): 381 601 738 911 1037 1123 1152 1139 1068 DOL(μm): 25.18 24.85 25.90 26.98 28.10 27.81 25.83 22.11 18.07

TABLE 4 Examples of ion-exchangeable glass compositions containing amixture of MgO and CaO. The compressive stress (CS) and depth of layer(DOL) were obtained as a result of treatment of annealed samples at 410°C. for 16 hours in technical grade KNO₃. Composition (mol %) 31 32 33 3435 36 37 38 39 40 SiO₂ 59.85 59.81 59.73 59.91 60.11 59.93 60.08 60.0061.92 63.96 A1₂O₃ 15.97 16.02 16.00 15.99 15.96 15.98 15.99 15.99 15.1814.39 Na₂O 15.85 15.70 15.75 15.79 15.82 15.83 14.67 13.86 15.00 14.21K₂O 1.07 1.87 MgO 1.65 3.41 5.14 5.76 6.39 7.33 5.65 5.74 5.46 5.14 ZnOCaO 6.52 4.90 3.23 2.40 1.57 0.78 2.38 2.39 2.28 2.14 SrO BaO 0.16 0.160.15 0.16 0.16 0.16 0.16 0.16 0.16 0.15 Properties 31 32 33 34 35 36 3738 39 40 Anneal Pt 677 681 683 686 692 695 684 685 689 693 (C): StrainPt (C): 629 632 633 637 642 645 635 635 639 641 Density 2.516 2.507 2.52.496 2.492 2.49 2.50 2.495 2.485 2.472 (g/cm{circumflex over ( )}3):CTE (×10{circumflex over ( )}- 84.5 82.9 82.4 81.8 81.7 81.60 84.10 85.278.9 76.5 7/C): Softening Pt 889 901 910 914 919 922 917 920 923 934(C): 24-h Liquidus 1150 1160 1150 1160 1190 1240 1160 1185 1165 1160(C): Primary Devit nepheline nepheline nepheline forsterite forsteriteforsterite forsterite forsterite forsterite forsterite Phase: LiquidusVisc 37523 49496 58629 53212 13399 58464 38735 63867 98439 (Poise):Poisson's 0.229 0.225 0.222 0.229 0.226 0.225 0.227 0.227 0.22 0.216Ratio: Shear 31.2 31.3 31.4 31.3 31.3 31.3 31.5 31.5 31.1 31.0 Modulus(GPa): Young's 76.8 76.6 76.6 77.0 76.7 76.7 77.2 77.3 76.0 75.3 Modulus(GPa): Refractive 1.5198 1.5177 1.5159 1.5151 1.5138 1.5132 1.51491.5149 1.5128 1.5102 Index: SOC 27.61 27.79 27.94 28.04 28.14 28.1627.93 27.94 28.38 28.68 (nm/cm/MPa): CS (MPa): 1168 1196 1210 1212 12021197 1140 1088 1172 1136 DOL (μm): 29.31 31.62 33.19 34.60 37.27 38.5241.24 45.69 36.11 38.15

TABLE 5 Examples of ion-exchangeable glass compositions containing SrOor BaO. The compressive stress (CS) and depth of layer (DOL) wereobtained as a result of treatment of annealed samples at 410° C. for 16hours in technical grade KNO₃. Composition (mol %) 41 42 43 44 45 46 4748 SiO2 76.26 67.91 60.22 52.01 75.99 67.70 60.27 51.86 Al2O3 0.03 7.9615.94 23.96 0.03 8.07 15.98 24.05 Na2O 15.58 15.90 15.72 15.87 15.6215.89 15.60 15.88 K2O MgO ZnO CaO SrO 7.99 8.09 7.98 8.02 BaO 8.21 8.177.99 8.05 SnO2 0.14 0.14 0.14 0.15 0.16 0.17 0.16 0.16 Properties 41 4243 44 45 46 47 48 Anneal Pt (C): 501 565 659 773 470 539 633 774 StrainPt (C): 460 520 610 721 431 496 585 725 Density (g/cm{circumflex over( )}3): 2.605 2.635 2.646 2.66 2.732 2.75 2.75 2.764 CTE (×10{circumflexover ( )}-7/C): 96.8 91.2 90 81 100.7 93.90 87.60 86.3 Softening Pt (C):676 751 867 648 722 849 24-h Liquidus (C): 990 1000 1165 >1390 980 8601240 1360 Primary Devit Tridymite Unknown Unknown Unknown TridymiteAlbite Unknown Unknown Phase: Liquidus Visc 7898 33812 19965 <1645 5587517914 10 3545 (Poise): Poisson's Ratio: 0.225 0.23 0.229 0.232 0.2280.23 0.23 Shear Modulus 27.65 29.59 31.10 32.28 26.53 28.66 31.67 (GPa):Young's Modulus 67.77 72.77 76.46 79.57 65.16 70.66 77.92 (GPa):Refractive Index: 1.5150 1.5212 1.5251 1.5310 1.5242 1.5296 1.53261.5370 SOC 26.82 26.62 26.73 25.92 25.43 25.44 25.74 26.35 (nm/cm/MPa):CS (MPa): 695 1137 1093 571 1053 1040 DOL (μm): 21.46 20.43 18.41 17.4015.71 18.18

TABLE 6 Analyzed compositions and selected properties ofboroaluminosilicate glasses in which the ratio [SiO₂]/[Al2O₃] wasmodified. The iron redox ratio was determined by ⁵⁷Fe Mössbauerspectroscopy on iron-containing glasses. log Hardness Glass Chemicalcomposition (mol %) CS N4 Diffusivity [Fe³⁺]/[Fe]₁ H_(nano) ID SiO₂Al₂O₃ Na₂O B₂O₃ Fe₂O₃ As₂O₃ (MPa) (at %) (cm²/s) _(ot) (at %) (Gpa) Al0*79.4 0.3 14.6 4.9 0.9 0 390 n/a −10.63 n/a 7.25 Al1* 78.9 0.7 14.5 4.90.9 0 421.7 n/a −10.67 n/a 7.21 Al2.5* 77.4 2.2 14.6 4.9 0.9 0 451 n/a−10.74 94 7.74 Al5* 74.7 4.7 14.6 5 1 0 558.6 n/a −10.72 92 7.91 Al7.5*71.8 7.6 14.7 4.9 1 0 688.3 n/a −10.59 90 7.85 Al10* 68.9 10.3 14.8 5 10 789 n/a −10.46 81 7.85 Al12.5* 67.1 12.6 14.3 5 1 0 906.8 n/a −10.2678 7.78 Al15* 64.1 15.6 14.3 5 1 0 995 n/a −10.16 76 7.46 Al17.5* 62.317.9 13.7 5.1 0.9 0 1073.3 n/a −10.35 n/a 7.30 Al20* 61.1 19.4 13.6 50.9 0 1041.4 n/a −10.54 n/a 7.27 Al0 80.1 0.2 14.8 4.8 0 0.2 364.8 95−10.46 n/a 7.08 Al1 79.4 1.2 14.5 4.9 0 0.1 400.4 92 −10.55 n/a 7.19Al2.5 78.8 2 14.4 4.7 0 0.1 370.6 90 −10.57 n/a 7.45 Al5 78.1 4 13.6 4.20 0.1 445.8 87 −10.65 n/a 7.57 Al7.5 76.9 5.7 13 4.3 0 0.1 557 77 −10.64n/a 8.03 Al10 75.9 7.5 12.3 4.3 0 0.1 602.1 68 −10.52 n/a 7.92 Al12.5 7210.4 13.1 4.4 0 0.1 760.5 39 −10.31 n/a 7.92 Al15 69.2 12.7 13.5 4.6 00.1 869 17 −10.12 n/a 7.79 Al17.5 63 17.2 14.7 5 0 0.1 1058.9 0 −10.17n/a 7.27 Al20 60.5 19.6 14.7 5 0 0.1 1018 0 −10.42 n/a 7.35

The invention claimed is:
 1. An alkali aluminosilicate glass comprising:from about 16 mol % to about 20 mol % Al₂O₃, Fe₂O₃, and from about 15mol % to about 20 mol % of at least one alkali metal oxide R₂O selectedfrom the group consisting of Li₂O, Na₂O, K₂O, Rb₂O, and Cs₂O, whereinR₂O includes 15 mol % to 20 mol % Na₂O, 0 mol %>Al₂O₃ (mol %) −Na₂O (mol%)≥−4 mol %, and the alkali aluminosilicate glass is ion exchangeable.2. The alkali aluminosilicate glass of claim 1, wherein the alkalialuminosilicate glass comprises: from about 55 mol % to about 70 mol %SiO₂; from about 16 mol % to about 20 mol % Al₂O₃; Fe₂O₃; from 0 mol %to about 10 mol % B₂O₃; from 0 mol % to about 5 mol % Li₂O; from 15 mol% to about 20 mol % Na₂O; from 0 mol % to about 5 mol % K₂O; from 0 mol% to about 10 mol % MgO; and from 0 mol % to about 10 mol % ZnO.
 3. Thealkali aluminosilicate glass of claim 1, wherein 15 mol%≤Li₂O+Na₂O+K₂O≤20 mol %.
 4. The alkali aluminosilicate glass of claim1, further comprising at least one divalent metal oxide.
 5. The alkalialuminosilicate glass of claim 4, wherein the at least one divalentmetal oxide is at least one of MgO, CaO, BaO, SrO, and ZnO.
 6. Thealkali aluminosilicate glass of claim 5, wherein the alkalialuminosilicate glass contains 0 mol % B₂O₃.
 7. The alkalialuminosilicate glass of claim 1, wherein the alkali aluminosilicateglass is free of divalent metal oxides.
 8. The alkali aluminosilicateglass of claim 1, wherein the alkali aluminosilicate glass is ionexchanged and has a compressive layer extending from a surface of thealkali aluminosilicate glass into the alkali aluminosilicate glass to adepth of layer.
 9. The alkali aluminosilicate glass of claim 8, whereinthe compressive layer is under a compressive stress of at least 1 GPa.10. The alkali aluminosilicate glass of claim 9, wherein the compressivelayer is under a compressive stress of at least 1.1 GPa.
 11. The alkalialuminosilicate glass of claim 8, wherein the alkali aluminosilicateglass has a nanohardness of at least 7 GPa.
 12. The alkalialuminosilicate glass of claim 1, further comprising at least one finingagent.
 13. The alkali aluminosilicate glass of claim 12, wherein thefining agent comprises at least one of SnO₂, As₂O₃, and Sb₂O₃.
 14. Thealkali aluminosilicate glass of claim 1, wherein the alkalialuminosilicate glass is free of divalent metal oxides other than MgOand ZnO.
 15. The alkali aluminosilicate glass of claim 1, wherein thealkali aluminosilicate glass has a liquidus viscosity in a range of atleast about 20 kiloPoise to 64 kiloPoise.
 16. The alkali aluminosilicateglass of claim 1, comprising about 1 mol % Fe₂O₃.
 17. The alkalialuminosilicate glass of claim 1, wherein the Al₂O₃ is present intetrahedral configuration.
 18. A touch-screen device or hand heldelectronic device comprising the alkali aluminosilicate glass of claim1.